An ideal current source has infinite output impedance and, as a result, provides a constant current over a wide operating voltage range. However, in reality, current sources have finite output impedance and limited output voltage swing. Furthermore, in low voltage applications, a low compliance voltage, Vcompl, may be desired to minimize the output voltage overhead.
A commonly used solution to maximize the output impedance involves the use of a regulated cascode current source. The regulated cascode current source offers the desired high output impedance. Another solution involves the use of an operational amplifier to enhance the regulated cascode current source. The use of the operational amplifier reduces the compliance voltage, Vcompl, which can make the design more suitable for low voltage applications.
One disadvantage of the prior art is that the regulated cascode current source suffers from a high compliance voltage, Vcompl, the voltage needed to avoid triode region operation. The high value of the compliance voltage, Vcompl, can prevent the use of the regulated cascode current source in low voltage applications.
A second disadvantage of the prior art is that the operational amplifier enhanced regulated cascode source requires a high gain operational amplifier, which can increase component count and overall limit on the bandwidth of the current source.